This invention relates to data processing systems of the kind having a plurality of data processing units (referred to herein as processing nodes) having access to shared data, common to two or more nodes. The invention is applicable both to multi-computer systems, in which the nodes are substantially independent computers, and to multi-processor systems in which the nodes interact co-operatively to perform programs and share common resources such as input/output devices.
In a conventional multi-processor system, the shared data is held in a common store, accessible to all the nodes. Each node may also have its own private store for holding non-shared data unique to that node. A problem with such a system is that the nodes must compete for access to the shared store and hence there may be conflict between different nodes attempting to access the shared store simultaneously. Moreover, there may be significant transmission delays between the nodes and the shared store. As a result, access to the shared data may be very slow.
U.S. Pat. No. 3,889,237 describes a dual processor system in which each processor has its own store and each store contains a duplicate copy of the shared data. To ensure that both copies are kept consistent, each processor has direct access to the store of the other processor so that it can write a new value of the shared data into both stores simultaneously. A problem with this prior proposal is that there may be conflict between the processors if both attempt to access the same store unit at the same time, and each processor must wait for all writes to the shared data to be completed before it can continue processing. This seriously reduces the efficiency of the system. Moreover, in this prior proposal, the shared data must be held in a fixed set of locations in each store, and this can prevent the efficient allocation of storage space in the processors. Both these problems make it very difficult to extend this proposal to more than two processors.
One object of the present invention is to alleviate these problems.